LTC3406B-2 Linear Technology, LTC3406B-2 Datasheet - Page 8

• Manufacturer Part Number: LTC3406B-2
• Description: (LTC3406B-1/-2) 600mA Synchronous Step-Down Regulator in ThinSOT
• Manufacturer: Linear Technology

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LTC3406B-1.2
APPLICATIO S I FOR ATIO
Typically, once the ESR requirement for C
met, the RMS current rating generally far exceeds the
I
mined by:
where f = operating frequency, C
and ∆I
voltage, the output ripple is highest at maximum input
voltage since ∆I
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice is
the AVX TPS series of surface mount tantalum. These are
specially constructed and tested for low ESR so they give
the lowest ESR for a given volume. Other capacitor types
include Sanyo POSCAP, Kemet T510 and T495 series, and
Sprague 593D and 595D series. Consult the manufacturer
for other specific recommendations.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. Because the
LTC3406B-1.2’s control loop does not depend on the
output capacitor’s ESR for stable operation, ceramic ca-
pacitors can be used freely to achieve very low output
ripple and small circuit size.
However, care must be taken when ceramic capacitors are
used at the input and the output. When a ceramic capacitor
is used at the input and the power is supplied by a wall
adapter through long wires, a load step at the output can
induce ringing at the input, V
couple to the output and be mistaken as loop instability. At
worst, a sudden inrush of current through the long wires
can potentially cause a voltage spike at V
to damage the part.
8
RIPPLE(P-P)
4
∆
V
OUT
L
= ripple current in the inductor. For a fixed output
≅ ∆
requirement. The output ripple ∆V
I ESR
L
L
⎛
⎜
⎝
increases with input voltage.
U
+
8
U
fC
1
OUT
IN
. At best, this ringing can
OUT
⎞
⎟
⎠
W
= output capacitance
IN
, large enough
OUT
OUT
U
has been
is deter-
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power times 100%. It is
often useful to analyze individual losses to determine what
is limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC3406B-1.2 circuits: V
and I
the efficiency loss at very low load currents whereas the
I
load currents. In a typical efficiency plot, the efficiency
curve at very low load currents can be misleading since the
actual power lost is of no consequence as illustrated in
Figure 2.
2
R loss dominates the efficiency loss at medium to high
Efficiency = 100% – (L1 + L2 + L3 + ...)
2
R losses. The V
0.0001
0.001
0.01
0.1
1
Figure 2. Power Loss vs Load Current
0.1
1
IN
LOAD CURRENT (mA)
quiescent current loss dominates
10
100
IN
V
V
V
IN
IN
IN
3406B12 F02
= 2.7V
= 3.6V
= 4.2V
quiescent current
1000
sn3406b12 3406b12fs